Communication is a basic function in which intelligence is transmitted from the transmitting location to a receiving location. Specific implementations of this function are many and varied depending upon the particular needs and rate of information transmission that is desired. One of the many trade-offs that can be made in designing communication apparatus is a function of the error rate and simply recognizes that the higher the allowable error rate the simpler the apparatus and conversely, lowering the error rate generally requires further complexity. A further constraint that is required in some applications is that the apparatus must be fail-safe, that is, the presence of a failure in the functioning of any of the components in the system must not be capable of bringing about a condition that is more dangerous than the condition that would have existed had the component not failed. For example, in a railroad application, wherein allowable speed information is the intelligence to be communicated, a communication failure should not allow a restrictive speed indication to be interpreted by a receiving apparatus as an indication of a higher allowable speed.
Apparatus of this sort has, in the past, employed analog techniques, and more particularly, has employed a so-called rate coding technique wherein the information takes the form of a tone or frequency modulated onto a carrier.
With the advent of small, low-cost and low-power-drain digital integrated circuits, there is a desire to employ these components in a communication system, and more particularly, in a fail-safe communication system. The prior art exhibits a number of digital communication systems having varying bit error rates. However, there are few, if any, digital communication systems which can be regarded as fail-safe. It is therefore one object of the present invention to provide a vital digital communication system.
It is another object of the invention to provide such a system wherein the digital logic, hard-wired or microprocessor provided, is checked by vital circuits before the output is allowed.
It is another object of the invention to provide a system with the foregoing features which will not produce an erroneous output from the decoder regardless of any failure, except in a vanishingly small number of unlikely situations.